This invention relates generally to radio antennas, and more particularly to improvements in a special form of antenna, known as a field probe. A field probe is used for measuring the strength of a radio frequency field. Field probes are particularly useful in the calibration of electromagnetic susceptibility testing equipment, in which an electronic device under test is exposed to a strong electromagnetic field swept through a range of frequencies. The invention relates specifically to improvements in E-field probes.
An E-field probe for electromagnetic susceptibility testing generally comprises one or more short antennas mounted on a housing containing processing circuitry. To minimize distortion of the antenna pattern resulting from the presence of conductive feeder cables, the processing circuitry typically includes a transducer which converts a detected r.f. voltage to a modulated light beam, which is then conducted away from the probe through a fiber optic cable. A fiber optic cable may also be used to deliver D.C. operating power to the processing circuitry in the form of a laser beam.
An E-field probe is typically in the form of single axis probes or a three axis probe. In either case, it preferably utilizes one or more dipole antennas. Even though fiber optic light conductors are used to minimize distortion, for most probes, especially those operable over a range of frequencies including frequencies in the gigaHertz region, it is not practical to position the processing circuitry at the dipole center. Accordingly the conventional practice has been to connect a conductive feeder, i.e., a transmission line from the processing circuitry to the dipole center (or, in the case of a multi-axis probe, to the dipole centers). The feeder itself causes distortion of the antenna pattern. In conventional practice, the distortion caused by the transmission line is reduced by using a high resistance feeder.
Conventional E-field probes also tend be highly frequency dependent in practice, and require compensation in their associated electronic circuitry in order to be useful over a broad frequency range. In addition, conventional E-field probes, tend to exhibit asymmetry in their antenna patterns. Asymmetry exists with respect to an imaginary plane to which the dipole elements are perpendicular (orientation asymmetry) and with respect to arbitrarily selected imaginary planes in which the dipole elements lie (axial asymmetry).
Still another drawback of conventional E-field probes is their tendency to be affected by stray fields, including magnetic fields.
An important object of this invention, therefore, is to avoid one or more of the aforementioned drawbacks of conventional probes.
Briefly, the preferred E-field probe in accordance with the invention, comprises one or more dipoles, each comprising plural, parallel, strings of discrete resistors mounted on a section of printed circuit board and connected electrically so that the parallel strings are twisted about each other for optimum orientation symmetry and minimum stray field pick-up. The values of the individual resistors are chosen so that the dipole is essentially a tapered resistance dipole, providing good performance over a very broad range of frequencies.
A particularly desirable feature of the invention is the provision of a pair of diodes connected respectively in parallel strings of one arm of the dipole, but connected electrically to each other in the same direction, i.e., having a direct connection between the anode of one of the diodes and the cathode of the other diode. This arrangement of diodes allows the feeder to be constituted by a pair of parallel series of resistors of comparatively high resistance value continuing from the outer end of the arm in which the diodes are situated. In this way, the dipole is effectively xe2x80x9cend fedxe2x80x9d rather then center fed, and pattern distortion and axial asymmetry are minimized.
In accordance with one aspect of the invention, the field probe comprises at least one elongated element at least part of the length of which is composed of a plurality of closely coupled conductors, preferably two, disposed in substantially parallel, side-by-side relation to one another. As used herein, the term xe2x80x9cclosely coupledxe2x80x9d with reference to parallel conductors means that the conductors are all situated within a cross-sectional area transverse to their direction of elongation, the largest dimension of which is much less than, i.e., not more than about one twentieth of, one wavelength at the highest frequency of intended operation. The highest frequency of intended operation is the maximum frequency where the antenna pattern (both in the E and H planes) has not significantly deviated from the pattern of an electrically short dipole. The term xe2x80x9cparallel, side-by side relation,xe2x80x9d when used with reference to conductors (including resistor strings) means that the conductors are not only parallel, but also that each end of one conductor is located adjacent an end of each other conductor.
In a practical electrically short probe, the conductors may be considered closely coupled when the maximum dimension of a transverse cross-section containing the conductors is within the range of approximately 0-15% of the length of the dipole portion of the probe. For a probe having a highest frequency of intended operation of 4.2 GHz, the largest dimension of the cross-sectional area in which the parallel conductors are located should be about 3.5 mm.
The elongated element is preferably a dipole antenna composed of two conductive elements, i.e., arms, extending in opposite directions from a dipole center at an intermediate location. At least one of the conductive elements is constituted by said part of the length of said at least one elongated element and comprises two closely coupled conductors disposed in substantially parallel, side-by-side relation to each other. These conductors extend from the intermediate location to an end location. Two terminals are provided at the end location, one terminal being at an end of one of the two closely coupled conductors and the other terminal being at an adjacent end of the other of the two closely coupled conductors. The probe includes processing circuitry and a transmission line having first and second opposite ends. The first end of the transmission line is connected to the processing circuitry, and the second end of the transmission line is connected directly to the dipole antenna at the terminals. The two closely coupled conductors are connected to each other at the intermediate location by a pair of diodes having a connection joining the diodes in series in the same direction electrically, and the other of the conductive elements is directly joined to said connection.
In a preferred embodiment, the conductors are resistive elements. Preferably, each resistance consists of plural discrete resistors connected in series. The discrete resistors preferably increase progressively in resistance with distance from the intermediate location on the dipole. In the case of two closely coupled resistances, the resistances are preferably twisted about each other.
If each of the closely coupled resistances is composed of two series of discrete resistors, the twisting of the resistances about each other is preferably achieved by situating alternate resistors of each series on opposite faces of a printed circuit board. The resistors on each side of the printed circuit board are disposed in a column parallel to the direction of elongation of said elongated dipole element, with the pairs of terminals of the resistors on each side of the printed circuit board situated in parallel lines in oblique relation to the direction of elongation of the elongated element. The resistors of each series may then be connected to one another by substantially straight conductors extending through the printed circuit board in perpendicular relation to the faces of the printed circuit board.
As will appear from the following detailed description, the invention provides an E-field probe having one or more of the following desirable characteristics: low antenna pattern distortion, a high degree of frequency independence, good symmetry, and stray field immunity.
Various other objects, details and advantages of the invention will be apparent from the following detailed description when read in conjunction with the drawings.